Single-site anionic polymerization. Monomeric ester enolaluminate propagator synthesis, molecular structure, and polymerization mechanism

The synthesis and molecular structure of the first examples of monomeric lithium ester enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids, are reported. Reactions of isopropyl alpha-lithioisobutyrate (2) with suitable deaggregating and stabilizing organoaluminum compounds such as MeAl(BHT)2 (BHT = 2,6-di-tert-butyl-4-methylphenolate) in hydrocarbons cleanly generate lithium ester enolaluminate complexes such as Li+Me2C=C(OiPr)OAlMe(BHT)2]- (3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and highly active polymerization can be achieved by using the 3/MeAl(BHT)2 propagator/catalyst pair, which is conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT)2. The structure of the added organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)2 pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator 2, and monomeric propagator 3 have been determined by X-ray diffraction studies.